Method for growing single thin film crystals



United States Patent 3,336,159 METHOD FOR GROWING SINGLE THIN FILMCRYSTALS Sidney H. Liebson, Dayton, Ohio, assignor to The National CashRegister Company, Dayton, Ohio, a corporation of Maryland No Drawing.Filed Oct. 7, 1963, Ser. No. 314,541

' 5 Claims. (Cl. 117-201) ABSTRACT OF THE DISCLOSURE A method of makinga planar single crystal layer by providing a crystalline material layerin which individual crystals of small dimension may be identified,thereafter building a crystal structure in conformity with a chosencrystallite of the layer, by melting and causing the flow of adjacentcrystals to make contact with the chosen crystal, followed by moving thezone of melting outwardly, as by the use of electron bombardment.

The present invention relates to a method for growing thin film crystalsand, more specifically, to a method for growing single thin filmcrystals of selected material and orientation upon amorphous substrates.

Recent developments in molecular electronics have been in the area ofelectric devices which may be prepared by diffusion techniques in singlecrystal silicon slices and the epitaxial growth of silicon on singlecrystal silicon slices.

For devices which are prepared by diffusion techniques in single crystalsilicon slices, it has been found that the electrical characteristics ofthe various diffused layers are interrelated. In view of this, theepitaxial growth of silicon crystals on single crystal silicon sliceshas been developed and affords the distinct advantage that theelectrical properties of the epitaxially deposited layer aresubstantially independent of those of the substrate, which may be apreviously deposited epitaxial layer.

Epitaxial growth of crystals, however, does have some limitations. Bytheir nature, crystals prepared by this technique cannot be grown to anarbitrary size, since the size is limited to the maximum size of thecrystal substrate which can be produced economically. With currentproduction techniques, the maximum crystal size is approximately fourcentimeters in diameter. Furthermore, the epitaxial growth techniquecannot be applied for the growth of crystals upon amorphous substratesbecause of the lack of crystalline structure of substrates of this type.

Truly three-dimensional circuitry could be prepared by growingadditional circuitry over the passivated surfaces of planar integratedcircuits if single crystal thin film silicon could be deposited upon anamorphous substrate, in that, currently, these passivated surfacesconsist of an amorphous silicon oxide or silicon dioxide layer, whichprecludes the epitaxial crystal growth technique. Therefore, a techniquewhich provides for the growth of single crystals upon such amorphoussubstrates would be a considerable advancement in the art, in that thecrystal size would be limited only by the size of the substrate.

It is, therefore, an object of this invention to provide a novel methodfor the growth of thin film single crystals.

It is another object of this invention to provide a novel method for thegrowth of thin film single crystals upon an amorphous substrate.

In accordance with this invention, a film of silicon which has beendeposited upon an amorphous substrate member by conventional techniquesis scanned, by suitable means, as explained more fully hereinafter, toidentify crystallites having certain preferred orientations with respectto the substrate, which crystallites, save a selected one, are then ICCmelted by an annular movement of an electron beam, the diameter of whichis increased while the power density of the beam is altered to maintaina molten zone within the confines of the beam, and the growth iscontinued by expanding the sweep of the annular beam until the substrate is completely covered by a single thin film crystal of silicon.

For a better understanding of the present invention, reference is madeto the following description.

In the following description, the unique method of this invention willbe described on the basis of depositing a thin film crystal of siliconupon an amorphous substrate such as -silicon oxide or silicon dioxide.It is to be specifically understood, however, that these materials areexemplary only for the purpose of accurately describing this noveltechnique and that this technique may be employed with other crystallinematerials and substrate members. In this regard, it is only necessarythat the film material be of a crystalline nature which may be organic,inorganic, metallic, or semi-conductor materials such as germanium,gallium phosphide, or indium antimonide, for example.

With the initial step of this novel method, an amorphous substrate suchas silicon oxide or silicon dioxide is coated with a film of crystallinematerial such as silicon by conventional techniques well known in theart, such as evaporation, disproportionation reaction, or hydrogenreduction. As these techniques are well known in the art, and any ofthem are acceptable for depositing the silicon layer upon the amorphoussubstrate, they will not be described in detail herein.

It is important, however, that care be exercised during this originaldeposition to achieve a polycrystalline layer of the largest size ofcrystals which can be conveniently attained.

After the film of silicon has been deposited upon the amorphoussubstrate, a single crystallite thereof having the desired orientationwith respect to the substrate is selected as the seed from which thesingle crystal will be grown. As is well known in the crystallographyart, the individual crystallites of silicon upon the amorphous substrateare randomly oriented relative to the surface thereof. For any specificapplication, there usually is a preferred crystal orientation. Forexample, a crystal orientation may be eX- pressed by the Miller indices1:1:1 for a specific application. Other applications may requiredifferent crystal orientation. After the preferred crystal orientationfor a specific application has been determined, the surface of thesubstrate is scanned by X-ray or electron diffraction to identifycrystallites deposited on the substrate which have certain preferredorientations, as dictated by the desired final result, and one chosenfrom among them.

When a satisfactory crystallite has been identified and located, thisselected seed crystallite is centered within a source of heat having anannular pattern. This heat source may be but is not limited to anelectron beam heating apparatus. The electron beam heating apparatus mayconsist of a simple electron beam gun operating in a vacuum anddeflected in a manner to produce a circular sweep, thereby providing asource of heat having an annular pattern. This equipment is commerciallyavailable, and the circular sweep deflection techniques are well knownin the cathode ray deflection art.

Dependent upon substrate temperatures, material used, thickness ofsubstrate, and thickness of the initial film, the electron beam voltageand current are adjusted to increase the intensity of the source of heatto provide an annular melting of the film about the selectedcrystallite. Because the beam diameter will in all probability begreater than the film thickness, it is important that these parametersbe varied to minimize separating effects of surface tension in themolten film within the annular area and the probabilities of dendriticgrowth formation within the molten zone.

After the intensity of the annular electron beam heating pattern hasbeen adjusted to the melting point of the deposited film of silicon, theannular electron beam heating pattern radius is slowly increased, andthe power density is simultaneously varied to increase the intensity ofthe source of heat to maintain the molten zone within the area of theannular electron beam heating pattern. The annular-like beam heatingpattern is then constantly expanded, the beam intensity always beingmade sufficient to bring about melting of the crystalline material Wherethe beam is incident. The moving beam leaves in its wake a cooling meltpath which crystallizes according to the pattern of the seed crystal inabutment thereto. This results in the growth of a single crystal ofsilicon over the entire surface area of the amorphous substrate member.

In the event that an unwanted crystal structure is detected within themolten area, the procedure is reversed, and the entire surface is meltedback to the point that the undersized structure is eliminated, and thenthe crystal growth is continued therefrom by expanding the annular beampattern in a manner thus previously described.

While a preferred embodiment of the present invention has been shown anddescribed, it will be obvious to those skilled in the art that variousmodifications and substitutions may be made without departing from thespirit of the invention, which is to be limited only within the scope ofthe appended claims.

What is claimed is:

1. A method for growing single thin film crystals upon an amorphoussubstrate comprising the steps of depositing a film of crystallinematerial upon an amorphous substrate, selecting a crystallite of thefilm of the desired orientation as the seed from which a single crystalwill be grown, centering the selected crystallite within a source ofheat having an annular pattern, adjusting the intensity of the source ofheat to provide an annular melting of the film about the selectedcrystallite, and simultaneously increasing the radius of the annularheating pattern and the heating intensity thereof to maintain a moltenzone within the area of the annular heating pattern until the entiredeposited film upon the substrate has been melted to produce a singlecrystal of the deposited crystalline material layer over the entiresurface of the substrate.

2. A method for growing single thin film crystals upon an amorphoussubstrate comprising the steps of depositing a film of silicon upon anamorphous substrate, selecting a crystallite of the film of the desiredorientation as the seed from which a single silicon crystal will begrown, centering the selected crystallite within a source of heat havingan annular pattern, adjusting the intensity of the source of heat toprovide an annular melting of the silicon film about the selectedcrystallite, and simultaneously increasing the radius of the annularheating pattern and the heating intensity thereof to maintain a moltenzone within the area of the annular heating pattern until the entiredeposited silicon film upon the substrate has been melted to produce asingle silicon crystal over the entire surface of the substrate.

3. A method for growing single thin film crystals upon an amorphoussubstrate comprising the steps of depositing a film of crystallinematerial upon an amorphous substrate, selecting a crystallite of thefilm of the desired orientation as the seed from which a single crystalwill be grown, centering the selected crystallite within a source ofheat comprising an electron beam deflected to produce an annular heatingpattern, adjusting the intensity of the electron beam annular heatingpattern to provide an annular melting of the film about the selectedcrystallite, and simultaneously increasing the radius of the electronbeam annular heating pattern and the heating intensity thereof tomaintain a molten zone within the area of the electron beam annularheating pattern until the entire de posited film upon the substrate hasbeen melted to produce a single crystal of the deposited crystallinematerial layer over the entire surface of the substrate.

4. A method for growing single thin film crystals upon an amorphoussubstrate comprising the steps of depositing a film of silicon upon anamorphous substrate, selecting a crystallite of the film of the desiredorientation as the seed from which a single silicon crystal will begrown, centering the selected crystallite within a source of heatcomprising an electron beam deflected to produce an annular heatingpattern, adjusting the intensity of the electron beam annular heatingpattern to provide an annular melting of the silicon film about theselected crystallite, and simultaneously increasing the radius of theelectron beam annular heating pattern and the heating intensity thereofto maintain a molten zone within the area of the electron beam annularheating pattern until the entire deposited silicon film upon thesubstrate has been melted to produce a single silicon crystal over theentire surface of the substrate.

5. A method of making a planar single crystal layer from crystallinematerial susceptible to melting by electron beam bombardment, comprisingthe steps of (a) disposing on a heat-resistant substrate a layer ofselected crystalline material of the same composition in whichindividual crystals of small dimension may be identified;

(b) choosing a small crystal of proper size around which an electronbeam may be directed, and

(c) directing an electron beam close to the chosen crystal to melt andcause flow of adjacent crystals to make contact with the chosen crystal;and

(d) moving the beam in substantially circular motion that constantlyincreases in radius, to build a crystal structure in conformity to thechosen crystal, the beam intensity always being made sulficient to bringabout melting of the crystalline material where incident.

References Cited UNITED STATES PATENTS 2,813,048 11/1957 Pfann 148-162,816,050 12/1957 Hunter 1481.6 2,926,075 2/1960 Pfann 148-1.6 2,968,7231/1961 Steigerwald 65 2,992,903 7/1961 Imber 148-1.6 3,160,522 12/1964Heywang et al 1481.6

ALFRED L. LEAVITT, Primary Examiner.

A. ROSENSTEIN, Examiner.

1. A METHOD FOR GROWING SINGLE THIN FILM CRYSTALS UPON AN AMORPHOUSSUBSTRATE COMPRISING THE STEPS OF DEPOSITING A FILM OF CRYSTALLINEMATERIAL UPON AN AMORPHOUS SUBSTRATE, SELECTING A CRYSTALLITE OF THEFILM OF THE DESIRED ORIENTATION AS THE SEED FROM WHICH A SINGLE CRYSTALWILL BE GROWN, CENTERING THE SELECTED CRYSTALLITE WITHIN A SOURCE OFHEAT HAVING AN ANNULAR PATTERN, ADJUSTING THE INTENSITY OF THE SOURCE OFHEAT TO PROVIDE AN ANNULAR MELTING OF THE FILM ABOUT THE SELECTEDCRYSTALLITE, AND SIMULTANEOUSLY INCREASING THE RADIUS OF THE ANNULARHEATING PATTERN AND THE HEATING INTENSITY THEREOF TO MAINTAIN A MOLTENZONE WITHIN THE AREA OF THE ANNULAR HEATING PATTERN UNTIL THE ENTIREDEPOSITED FILM UPON THE SUBSTRATE HAS BEEN MELTED TO PRODUCE A SINGLECRYSTAL OF THE DEPOSITED CRYSTALLINE MATERIAL LAYER OVER THE ENTIRESURFACE OF THE SUBSTRATE.